1. Field of the Invention
The present invention relates to a direct methanol fuel cell that comprises a material layer for controlling the diffusion rate of fuel, and thus has reduced methanol crossover. More particularly, it relates to a direct methanol fuel cell exhibiting high power density while using high-concentration methanol fuel, in which a layer of material for controlling the diffusion rate of fuel is interposed between an anode (fuel electrode) and a fuel reservoir.
2. Description of the Prior Art
Fuel cells are cells that convert chemical energy resulting from the oxidation of fuel directly into electrical energy. Unlike general batteries, fuel cells are a power generation system that can continue to make electricity as long as fuel is supplied, without the need for recharging. The fuel cell is a structure in which an electrolyte and two electrodes are stacked in a sandwich configuration. In the fuel cell, when hydrogen and methanol flow toward their electrodes, electricity, heat and water will result.
The fuel cell is a field in which concerned technology institutes and enterprises are very interested as a new electricity generation system. The fuel cell is a kind of direct current power generation system that converts the chemical energy of fuel directly into electrical energy through electrochemical reaction, and has advantages of high power generation efficiency and reduced problems caused by hazardous exhaust gases, such as NOx and CO, compared to other power generation systems, such as diesel power generation systems and gas turbine systems.
The fuel cells can be divided, according to the kind of electrolyte used, into a polymer electrolyte membrane (PEM) type, a phosphoric acid type, a molten carbonate type and a solid oxide type. Among them, the polymer electrolyte membrane-type fuel cell has characteristics of low operating temperature, high efficiency, high current density and output density, short starting time, and quick response to load change, compared to the other types of fuel cells. In addition, since it utilizes the polymer membrane as an electrolyte, it has advantages in that corrosion and the electrolyte do not need to be controlled, it has a simple design, it is easily manufactured, and it has a smaller volume and weight than those of the phosphoric acid-type fuel cell, having the same operating principle.
The polymer electrolyte fuel cells can be divided, according to the type of fuel, into a cell utilizing gas such as hydrogen, and a cell utilizing liquid such as methanol. The polymer electrolyte fuel cell utilizing hydrogen fuel provides an advantage of high energy density, but requires caution in the storage and transport of hydrogen gas. Also, since it requires a separate reformer system for obtaining hydrogen gas, it will have many problems to be solved, when it is applied to a portable cell or a small-sized power generation system.
On the contrary, a direct methanol fuel cell is useful as a portable power generation system, because liquid methanol is injected therein without the need for a separate reformer system. However, the direct methanol fuel cell has shortcomings in that it has a low energy density and requires a large amount of a precious metal catalyst, compared to a hydrogen cell. Nevertheless, considering that the direct methanol fuel cell realizes easy handling of fuel and is inexpensive, it is expected to have very high utility as a small-sized power source.
Lithium-ion cells and lithium polymer cells, which are currently widely used as portable power sources, have energy densities of about 150 Wh/kg and 300 Wh/kg, respectively, whereas the direct methanol fuel cell can achieve an energy density reaching about 1,000 Wh/kg. Furthermore, while the lithium secondary cells require a charge time of about 3 hours, the methanol fuel cell requires a time of only a few seconds to inject aqueous methanol solution fuel therein, and thus has a great advantage in terms of charge time.
The direct methanol fuel cell is in the form of a membrane electrode assembly (MEM) generally comprising catalyst layers (i.e., an anode and a cathode) for oxidizing and reducing fuel, and a polymer electrolyte. All electrochemical reactions consist of two separate reactions that are oxidation at an anode and reduction at a cathode (air electrode), in which the anode and the cathode are separated from each other by an electrolyte. In the direct methanol fuel cell, methanol and water are supplied to the anode, and hydrogen ions generated during the oxidation of methanol move to the cathode along the polymer electrolyte to reduce oxygen supplied to the cathode so as to generate electricity. Outside the two catalyst layers, fuel diffusion layers functioning to effectively supply fuel and as current collectors are disposed.
Despite the above-mentioned various advantages, the direct methanol fuel cell has several problems that must be solved to realize practical application. The methanol crossover phenomenon, one of these problems, is the phenomenon by which methanol crosses over the electrolyte membrane to the cathode without complete oxidation when methanol is injected in a liquid state into the anode, and is the biggest problem to be solved in the direct methanol fuel cell. Due to the methanol crossover phenomenon, the direct methanol fuel cell suffers a fuel loss of more than 20% and a voltage loss of more than 0.1 V.
U.S. Pat. No. 6,866,952 makes an attempt to suppress the crossover of methanol by interposing between two conductive polymer membranes a barrier having micropores smaller than methanol molecules.
Also, U.S. Pat. No. 6,296,964 makes an attempt to uniformly supply methanol into an anode while limiting the crossover of methanol by forming micropores in a gold-coated stainless steel plate and controlling the size and configuration of the micropores.
In addition, methods for suppressing the crossover of methanol by the following means were reported: using a barrier formed by coating tetra(orthoamino-phenyl)porphyrin on the surface of a cathode by electropolymerization (A. Bettelheim, L. Soifer, E. Korin, Journal of Electroanalytical Chemistry, 571 (2004) 265-272); and interposing a palladium foil between two sheets of hydrogen ion-conductive Nafion™ 117 polymer membrane so as to transfer only hydrogen ions and suppress the transfer of methanol through the palladium foil (C. Pu, W. Huang, K. L. Ley, E. S. Snotkin, Journal of Electrochemical Society, 142 (1995) L119-120).
Korean Patent Laid-Open Publication No. 10-2005-30455 discloses a method of improving the performance of cells by disposing on an anode a fuel diffusion layer having a micropore structure, thus uniformly supplying an aqueous methanol solution and making the discharge of carbon dioxide, produced in the reaction, smooth.